1. Field of the Invention
The invention relates generally to systems and methods for sensing axial loads in a drillstring. More particularly, the invention relates to systems and methods for sensing weight-on-bit and axial loads in a drillstring that provide reduced sensitivity to differential temperature, differential pressure and bending effects on the drillstring.
2. Background of the Technology
The axial loads and torque applied to a drill bit during the drilling of a well are important parameters affecting the direction and inclination of the borehole, drilling efficiency, the durability of the drill bit, as well as the economics of the drilling operation. In addition, determination of the axial loads and torques acting on the drill bit allow an operator to detect the onset of drilling problems and correct undesirable situations before a failure of any part of the system. Some of the problems that can be detected by measuring the axial loads and torques on the drill bit include motor stall, stuck pipe, and bottom hole assembly (“BHA”) tendency. By determining these forces, a drill operator is also able to optimize drilling conditions so a borehole can be drilled in the most economical way. Consequently, the axial loads and torques applied to a drill bit are carefully monitored and controlled during drilling operations.
The axial compressive load on the drill bit is often referred to as “weight-on-bit” or “WOB.” Weight is typically applied to the drill bit by a string of heavy drill collars that are attached above the drill bit and suspended in the borehole on a smaller diameter drillstring. In conventional drilling practice, the entire length of the drillstring and the upper portion of the drill collar are suspended at the surface in tension by a derrick so that the amount of WOB can be adjusted by changing the surface hook load. WOB is carefully controlled during drilling operations as it affects the rate of penetration (ROP) of the drill bit, the drill bit wear and the direction of drilling. The torque applied to the drill bit (“torque-on-bit” or “TOB”) is also important with regard to drill bit wear and drilling direction, particularly when considered together with measurements of WOB. Excessive TOB is indicative of serious bit damage such as bearing failure and locked cones.
Typically, measurements of WOB are made at the surface by comparing the “hook load weight” of the drillstring to the “off-bottom weight” of the drillstring, and measurements of TOB are made by measuring the torque applied to the drillstring at the surface. However, reliability of such surface measurements of WOB and TOB are a known problem as other forces acting on the drillstring downhole often interfere with surface measurement.
More recently, systems have been devised for taking measurements “downhole” and transmitting these measurements to the surface during the drilling of the borehole. Typically, such systems rely on one or more strain gauges coupled to the drillstring downhole proximal the drill bit. In general, a strain gauge is a small resistive device that is attached to a material whose deformation is to be measured. The strain gauge is attached in such a way that it deforms along with the material to which it is attached. The electrical resistance of the strain gauge changes as it is deformed. By applying an electrical current to the strain gauge and measuring the differential voltage across it, the resistance, and thus the deformation, of the strain gauge can be measured. However, such strain gauges are subject to significant inaccuracies because they may be deformed by means other than axial loads on the drillstring. For example, strain gauges may experience deformation due to bending of the drillstring, pressure differentials between the drilling mud within the drillstring and borehole pressure outside the drillstring, and temperature gradients. Unfortunately, strain gauges are not adept at distinguishing between strain due to axial loads versus axial strain induced by pressure differentials, temperature gradients, and bending.
Accordingly, there remains a need in the art for improved systems and methods for sensing axial loads on a drillstring and WOB. Such systems and methods would be particularly well-received if they were less susceptible to inaccuracies due to pressure differentials, temperature gradients, and bending of the drillstring.